This application claims the priority of German patent document 199 09 140.4, filed Mar. 3, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an electronic apparatus for determining the distance between two objects, and to an electronic security system equipped with such a distance-determining apparatus.
Electronic devices for measuring distance are known, for example in the form of propagation delay-measuring devices. In such instruments, a signal transmitted by one object (referred to herein as the interrogation signal) is reflected from the other object and returns as a reflected response signal to the signal-transmitting object. Thereafter, an evaluation unit determines the distance between the two objects based on a measured propagation time between transmission of the interrogation signal and receipt of the reflected response signal. Systems of this type, in which the response signal consists of a passively reflected signal, are described in the German patent documents DE 42 40 491 C2 and DE 44 07 369 A1.
German patent document DE 44 36 983 A1 discloses an apparatus of this type, which measures the distance between a vehicle and another object, e.g., a vehicle. A time-dilatated interrogation pulse generated by expansion of a temporally short interrogation pulse is emitted by one object and is received by the other object, where it is compressed back into the temporally short interrogation pulse. From the latter pulse a time-dilatated response pulse is generated and transmitted back to the first-mentioned object, where it is converted back into a short response pulse by compression. The time interval between the response pulse and the emitted short interrogation pulse is used as a measure of the distance between the two objects. The duration of the temporally short interrogation pulse and response pulse should be chosen to be short enough (e.g., a few nanoseconds) to allow a propagation delay of this type from the determination of the time interval between the two pulses for typical distances that are considered.
Modern motor vehicles, in particular passenger cars, are making increasing use of electronic security systems in the form of locking systems and/or electronic immobilizers which can be activated by the authorized vehicle user or users by means of an authentication element depending on the range or distance at which an authentication element that is valid for the relevant vehicle is situated relative to the vehicle. In so-called keyless-go systems, it is no longer necessary for the user to actively actuate the authentication element; rather, all that the user need do is carry the authentication element, which comprises a smart card for example, with him. Different security actions can be performed depending on where an associated identification sensor system detects an authorizing authentication element. A vehicle locking system of this type is described in the earlier German patent document DE 198 39 355, which was not published before the priority date.
In such locking systems and/or electronic immobilizers of vehicles (and in similar electronic object security systems) in which certain security actions are intended to be initiated depending on the distance between an authentication element and the object to be secured, there is a need to determine the distance between the authentication element and the object to be secured, in the simplest possible manner.
One object of the invention is to provide an electronic distance-determining apparatus of the type described above which, with relatively little outlay, makes it possible to determine even relatively short distances (as little as 1 m or even less).
Another object of the invention is to provide an electronic security system which is equipped with such a distance measurement apparatus, and can be designed, in particular, as a keyless-go system for a locking system and/or an electronic immobilizer of a motor vehicle.
These and other objects and advantages are achieved by the electronic distance-determining apparatus according to the invention, in which respective transmission and reception units are provided in both objects. The interrogation signal transmitted by a first of the two objects is received by the second object via its reception unit, after which the second object returns a response signal to the first object via its transmission unit. A suitable evaluation unit provided in the first object determines a phase difference between the emitted interrogation signal and the received response signal, which phase difference is indicative of the distance between the two objects, and from which the desired distance can then be determined.
By virtue of the active generation and emission of the response signal, the apparatus remains independent of the reflection properties of the object, in contrast to systems in which the response signal is merely a passively reflection from the relevant object. Moreover, by virtue of the evaluation of the phase difference between the interrogation and response signals, it is possible to determine even relatively short distances, on the order of 1 m or even less, with signals based on electromagnetic waves, without requiring that the interrogation and response signals be kept very short, in terms of time, for example by compression.
In one embodiment of the invention, a delay unit is provided in the first object, which internally delays the interrogation signal prior to the phase comparison with the returned response signal, by a period of time which corresponds to the signal processing time in the second object (that is, the time difference between the beginning of the reception of the interrogation signal and the beginning of the transmission of the response signal). This enables a correctly timed comparison between the interrogation signal and the response signal from the beginning of the distance-determining process, avoiding a phase difference between the interrogation and response signals caused by signal processing delay.
In another embodiment of the invention, a signal generation unit (triggered by the received interrogation signal) with a synchronizing PLL (phase-locked loop) circuit is provided in the second object to receive the interrogation signal, for the purpose of generating the response signal. This has the particular advantage of ensuring continual synchronization between the interrogation signal and the response signal, which avoids phase differences on account of frequency fluctuations of the signal generation unit that generates the response signal with regard to the interrogation signal.
According to another feature of the invention, the evaluation unit may comprise an exclusive-OR gate, to which the interrogation and response signals are fed via respective inputs. The exclusive-OR gate distinguishes the time periods with an identical signal level from those with a different signal level of the interrogation and response signals, and therefore generates an output signal which is representative of the phase difference between the two signals and can be further processed in a suitable manner. Such further processing may comprise, e.g., forming the root-mean-square value of the output signal of the exclusive-OR gate, after which the root-mean-square value that is generated represents a measure of the distance between the two objects.
In still another embodiment of the invention, such root-mean-square value formation is combined with a threshold value comparison unit. The threshold value comparison unit compares the root-mean-square value formed from the output signal of the exclusive-OR gate with a fixed or preferably variably predeterminable threshold value. The specification of the threshold value may serve for suppressing slight phase differences between the interrogation and response signals on account of signal processing tolerances of the various components of the apparatus, or for setting a threshold which corresponds to a distance threshold value, where the intention is to identify when the distance that is actually determined exceeds or falls below the distance threshold value. Thus, it is possible, for example, to ascertain whether the two objects are spaced apart from one another by more than a predeterminable maximum distance.
In still another embodiment, with an evaluating exclusive-OR gate, the further evaluation comprises charging a capacitor (provided for this purpose) with the output signal of the exclusive-OR gate. The rising capacitor voltage in the course of the charging process is compared with a fixed or preferably variably predetermined threshold value voltage in a threshold value comparison unit connected downstream. The time taken to charge the capacitor up to the threshold value voltage is then representative of a corresponding threshold value of the distance between the two objects. In this way, by way of example, a maximum distance can again be prescribed, and it is possible to ascertain whether or not the actual distance between the two objects exceeds the maximum distance.
Yet another embodiment of the invention permits the determination of relatively large distances, which correspond to phase differences between the interrogation and response signals of more than the width of an individual pulse of these signals that is typically used. (In such circumstances, the phase difference can no longer be unambiguously determined from a phase difference evaluation of a signal which comprises regularly successive individual pulses, owing to the periodicity.) To that end, the interrogation signal-transmitting object comprises a distance code word generation unit, which generates a predeterminable distance code word as a digital data word which is emitted as an interrogation signal, in this case for the purpose of determining a relatively large distance. The response signal which is generated as an identical data word response thereto in the other object is then checked by the evaluation unit with regard to a possible distance-dictated temporal offset relative to the interrogation signal data word. Even distance-dictated shifts between digital interrogation and response signals of more than 1 bit of the data word can be ascertained in this manner.
Finally, the invention also provides a security system which is equipped with a distance-determining apparatus of the type described above. With this system, it is possible to determine the distance between the authentication element (by which a user who carries it with him proves to the security system that he is authorized) and an object which is secured by one or more security units of the security system. In particular, the security system may be a locking system and/or electronic immobilizer of a motor vehicle designed as a keyless-go system. The determination of the distance between the authentication element and the object to be secured can be utilized, for example, to permit certain control commands for the security system only if the authentication element (and hence the system user who is carrying it with him) are within a predetermined distance of the object to be secured. It is also possible, as required, to define different distance ranges for different security actions of the security system.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.